The black fungus gnat species of the family Sciaridae, which characteristically have polytene chromosomes in several tissues, are excellent organisms in which to study genic activity, cellular and nuclear differentiation, host-parasite relationships, direct effects of virus-induced unicellular tumors, and changes in chromosomal structure and function, resulting from normal physiological, developmental, or infection processes; all of which are problems of fundamental importance to biology and biomedical sciences. Puffs which occur with specific periodicity in certain well-defined regions of the chromosomes are visualizations of genic activity. The analysis of events which occur on the polytene chromosomes of the same tissue at different stages of the larval life cycle or in different tissues has given us some insight into how genes operate during physiological and developmental processes. The use of biological agents, such as microsporidia, gregarine, and virus as inducers of change in cellular development and chromosomal structure and function was developed by members of our group and has been found to be most promising in the investigation of cell physiology and development in eukaryotes. The visualization of alterations in chromosomal structure as induced by virus in unicellular tumors of Sciaridae may bring us to a closer understanding of similar occurrences in cancer induction in man and other organisms. Furthermore, an understanding of the host-parasite relationship in the Sciaridae, coupled with the studies we hope to do on mutation induction in these infective agents would be of interest and benefit to those individuals involved with seeking a more suitable means of biological pest control. We are analyzing the problems outlined here in several ways: light microscopically, using stained and autoradiographic preparations; electron microscopically; electrophoretically and biochemically.